Selective deposition of various metals and metal alloys onto features made of copper or a copper alloy by electroless (autocatalytic) plating is a common method in the manufacture of printed circuit boards and IC substrates as well as in metallization of semiconductor devices in e.g. microchip manufacture. Palladium is an example of a metal to be deposited onto such copper or copper alloy features. Examples of metal alloys for this purpose are nickel alloys such as Ni—P alloys and cobalt alloys such as Co—Mo—P and Co—W—P alloys.
A copper or copper alloy surface requires an activation prior to the electroless (autocatalytic) deposition of a metal or metal alloy thereon. The most prominent means for activating a copper or copper alloy surface is the deposition of a palladium seed layer by immersion-type plating of palladium which is a redox reaction wherein copper is oxidized and palladium ions present in the activation solution are reduced to metallic state. Such a seed layer of palladium then serves as the plating base for depositing a metal or metal alloy layer onto the copper or copper alloy feature by electroless (autocatalytic) plating. The activation solutions utilized are usually acidic.
A method for activating a copper or copper alloy surface for successive electroless deposition of a metal or metal alloy thereon is disclosed in U.S. Pat. No. 7,285,492 B2. The substrate is first contacted with a cleaning solution comprising an aqueous solution of a carboxyl group-containing organic acid or its salt and a surfactant. Next, the cleaned substrate is contacted with a processing solution comprising palladium ions and a carboxyl group-containing organic acid or a salt thereof. A metal or metal alloy layer can then be deposited onto the activated substrate by electroless (autocatalytic) plating.
The immersion-type plating of palladium from an acidic solution causes the formation of undesired voids in the surface area of the copper or copper alloy feature activated. Such voids, for example, reduce the reliability of solder joints or wire bonds in case the copper or copper alloy feature is a contact pad used for soldering or wire bonding.